JPH04371527A - Production of high-strength cold rolled steel sheet for deep drawing - Google Patents

Abstract

PURPOSE:To provide the process for production of the high-strength cold rolled steel sheet for deep drawing by specifying Ts and r values and transition temp. CONSTITUTION:A steel having a steel compsn. consisting, by weight %, of <=0.0050% C, 0.3 to 1.5% Si, 0.5 to 2.5% Mn, <=0.100% P, <=0.030% S, 48/14XN to 2X48/14XN% Ti, >=(93/12XC%) Nb, Nb>=Ti%, <=0.0040% N, 0.010 to 0.090% Al, and the balance Fe and unavoidable impurities is subjected to hot rolling in a temp. region above the Ar3 point and thereafter, the steel sheet is coiled at <=50 deg.C. This steel sheet is subjected to cold rolling at >=50% draft and is annealed in a temp. region of 800 deg.C to Ac3 point.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high-strength cold-rolled steel sheets for deep drawing that have excellent resistance to secondary work brittleness.

0002

[Background Art] In recent years, when manufacturing automobiles, high-strength cold rolling is applied to the bodies of various automobiles, including passenger cars, in order to reduce vehicle weight, reduce fuel consumption, and improve vehicle dynamic performance. The use of steel plates has become common. Along with this, there is a strong demand for cold-rolled steel sheets to further increase their strength and deep drawability.

Of course, great efforts have been made to develop high-strength cold-rolled steel sheets for deep drawing that have high strength and further improved deep drawability, and various high-strength cold-rolled steel sheets for deep drawing have been developed. has been developed. However, according to the conventional common method of producing high-strength cold-rolled steel sheets for deep drawing by adding P, Si, Mn, etc. to ultra-low carbon steel, the resulting steel sheets suffer from secondary work brittleness. The problem was that it was very easy to occur.

[0004] Therefore, Japanese Patent Publication No. 1-28817 proposes a technique for obtaining good resistance to secondary work brittleness by adding a composite of B, Ti, and Nb.

[0005]

However, in the method disclosed in the above publication, the r value decreases due to the addition of B.

[0006] The object of the present invention is to produce a high-strength cold-rolled steel plate for deep drawing that has excellent resistance to secondary work brittleness and good deep drawability, specifically, a steel sheet with a TS of 43.0 kgf/m.
An object of the present invention is to provide a method for producing a high-strength cold-rolled steel sheet for deep drawing, which has mechanical properties of m2 or more, r value: 1.8 or more, and has a transition temperature of -90°C or less.

[0007]

[Means for Solving the Problem] As a result of various studies aimed at developing a high-strength cold-rolled steel sheet for deep drawing that has extremely good deep drawability and excellent resistance to secondary work brittleness, the present inventor has found that:
In ultra-low carbon high-strength cold-rolled steel sheets, deep drawability is improved by appropriately controlling the amount of Ti and Nb in the steel billet used, and by limiting the coiling temperature after hot rolling and the annealing temperature after cold rolling. The present invention was completed based on the finding that a high-strength cold-rolled steel sheet for deep drawing, which has good properties and is extremely resistant to secondary work brittleness, can be obtained without adding B.

[0008] Here, the gist of the present invention is that C
: 0.0050% or less, Si: 0.3-1.5%,
Mn: 0.5-2.5%, P: 0.100% or less, S
: 0.030% or less, Ti: 48/14 x N ~ 2 x 4
8/14×N%, Nb: 93/12×C% or more, and N
b≧Ti%, N: 0.0040% or less, Al: 0.0
A steel having a composition consisting of 10% to 0.090% Fe, the balance being Fe and unavoidable impurities is hot rolled in a temperature range above the Ar3 point, then coiled at 500°C or less, and then cold rolled to 800°C. This is a method for producing a high-strength cold-rolled steel sheet for deep drawing that has excellent resistance to secondary work brittleness and is characterized by annealing in a temperature range of .degree. C. to Ac3.

[0009]

[Action] The action and effect of the present invention will be explained.

First, the reason for limiting the composition and manufacturing conditions of the steel used in the present invention will be explained.

C: If the C content is high, the amount of Ti and Nb for fixing C must be increased,
Manufacturing costs will increase. Furthermore, the amount of precipitated carbides of Ti and Nb also increases, leading to deterioration of the properties of the material. Therefore, in the present invention, the C content is limited to 0.0050% or less.

Si: Si is actively added as a reinforcing element because it increases the strength of the steel sheet without reducing its ductility. Therefore, the lower limit is set at 0.3%. However, if added in excess, Si has the property of embrittling the steel plate, and specifically, if added in excess of 1.5%, the resistance to secondary work embrittlement deteriorates. Therefore, the upper limit is set at 1.5%. Therefore, in the present invention, the Si content is limited to 0.3% or more and 1.5% or less.

Mn: Mn combines with S to form MnS and prevents the formation of TiS and Ti4C2S2. When the Mn content is less than 0.5%, TiS and Ti4C2
S2 is produced and prevents the production of (Ti, Nb)C, which will be described later. Therefore, the lower limit is set at 0.5%. On the other hand, although Mn is effective in increasing the strength of steel, excessive addition deteriorates deep drawability. Therefore, the upper limit is 2.5
limited to %. Therefore, in the present invention, the Mn content is limited to 0.5% or more and 2.5% or less.

P: P is the element with the greatest strengthening ability and is added to increase strength, but if it is included in excess, the amount of grain boundary segregation increases, causing embrittlement, that is, secondary processing cracking. . Therefore, in the present invention, the P content is 0.
．． Limited to 100% or less.

[0015] S: If the S content is high, TiS is likely to be formed, and the formation of (Ti, Nb)C is hindered. Therefore, in the present invention, the S content is limited to 0.030% or less.

Ti: Ti is an important element along with Nb in the present invention, and its content is limited for the reasons listed below.

■In order to generate TiN and eliminate solid solution N that has an adverse effect on deep drawability, the lower limit of the Ti content is set to 4
It is limited to 8/14×N%.

(Ti, Nb)C is produced together with Nb. By producing (Ti, Nb)C, solid solution C is eliminated upon completion of hot rolling, and a texture advantageous for deep drawability is obtained in subsequent annealing. In addition, during annealing (800 to 900 °C) after cold rolling, (Ti, Nb)C
is re-dissolved to strengthen grain boundaries and improve secondary processing brittleness resistance. At this time, if Ti is excessive, precipitation of TiC will be promoted and solid solution of C will not occur even after annealing, making it impossible to obtain good resistance to secondary work brittleness. Therefore, the upper limit of Ti content was set to 2×48/1
4 Limited to N%.

Nb: If the Nb content is small, it is sufficient (T
i, Nb) Since C does not precipitate, solid solution C remains and a structure advantageous for deep drawability cannot be obtained after annealing. Therefore, the lower limit of the Nb content is limited to 93/12C%.

[0020] Also, the reason for setting Nb≧Ti is that Nb<Ti
This is because (Ti, Nb) Precipitation of TiC is promoted more than C, so C does not re-dissolve in the solid state even after annealing, and good secondary work embrittlement resistance cannot be obtained.

N: If the N content is large, the amount of Ti required to fix it will be large. Furthermore, the amount of TiN precipitated increases, which adversely affects ductility. Therefore, in the present invention, the N content is limited to 0.0040% or less.

Al: 0.010% or more of Al is necessary for deoxidizing molten steel, and addition of more than 0.090% will impair economic efficiency. In the present invention, the Al content is limited to 0.010% or more and 0.090% or less.

Compositions other than the above are Fe and inevitable impurities.

Hot Rolling and Coiling Temperature In the present invention, the steel having the above composition is hot rolled in the austenite single phase region and coiled at a temperature of 500° C. or less. This is because the r value after annealing is better when the winding temperature is low than when it is high, and this is a concern when performing low temperature winding by controlling the Ti and Nb amounts as described above. This is because it is possible to avoid residual amounts of solid solution C and N. Therefore, the upper limit of the winding temperature is limited to 500°C.

Annealing temperature The hot-rolled steel plate wound up as described above is cold rolled to a desired thickness, and then annealed at a temperature of 800° C. or more and Ac3 point or less. The annealing temperature must be higher than the recrystallization temperature, but even if it is higher than the recrystallization temperature, it is 800℃.
If it is less than (Ti, Nb), re-solid solution of (Ti, Nb)C is difficult to occur. Therefore, the lower limit of the annealing temperature is limited to 800°C. In addition, the upper limit of the annealing temperature is A
It is point c3. Note that if the annealing temperature exceeds 900°C, economic efficiency will be impaired due to problems such as the life of the furnace. Therefore,
In the present invention, the annealing temperature is 800 °C or higher Ac3
The temperature is preferably 800° C. or higher and 900° C. or lower.

[0026] It is not necessary to specify any particular cold rolling conditions. As the rolling reduction during cold rolling increases,
Deep drawability tends to improve, and secondary work embrittlement is less likely to occur as the r value (deep drawability) of the steel sheet increases, so it is desirable that the cold reduction ratio is 50% or more.

As described above, the present invention mainly allows C and N to be completely precipitated by Ti and Nb at the stage of hot-rolled steel sheet by limiting the amount of Ti and Nb to the range of the present invention.
By annealing after cold rolling, extremely good deep drawability is obtained, and by re-dissolving C after recrystallization and concentrating the solute C in the grain boundaries, good secondary work brittleness is obtained. be. Further, according to the present invention, the secondary work brittleness resistance is further improved by improving the deep drawability.

Next, the present invention will be explained in detail with reference to examples.

[0029]

[Example] A slab having the composition shown in Table 1 was melted, hot rolled at a finishing temperature of 900 to 940°C, coiled at the temperature shown in Table 1, and hot rolled to a thickness of 4.5 mm. A steel plate was produced, and then cold-rolled at a reduction rate of 78% to obtain a cold-rolled steel plate with a thickness of 1.0 mm.

Next, the obtained cold rolled steel sheet was annealed in a continuous annealing furnace. In the annealing cycle, the samples were heated at a rate of about 10° C./S to the temperature shown in Table 1, held at that temperature for 40 S, and then cooled to room temperature at a cooling rate of 40° C./S. Furthermore, the skin pass is 0.8
After multiplying by %, it was subjected to a material test.

Table 2 shows the "tensile properties" and "transition temperature" of the steel sheets produced in this manner. here"
"Transition temperature" means the boundary temperature at which brittle cracking occurs,
After drawing and forming a cylinder with a drawing ratio of 1.6, this was placed over a truncated cone, and the measurement was performed by applying an impact and pushing the cylinder in to check for brittle cracks.

[0032]

[Table 1]

[0033]

[Table 2]

[0034] All steel sheets manufactured satisfying the conditions of the present invention have extremely good deep drawability and exhibit excellent resistance to secondary work brittleness.

It can be seen that steel sheets manufactured without satisfying the conditions of the present invention do not have sufficiently satisfactory characteristics as high-strength cold-rolled steel sheets for deep drawing.

[0036]

As explained above, according to the present invention, it is possible to produce a steel plate having extremely excellent deep drawability and excellent resistance to secondary work brittleness. This brings about extremely useful effects industrially, such as being able to fully meet the strict requirements of steel sheets for automobiles.

Claims (1)

[Claims] Claim 1: In weight percent, C: 0.0050% or less, Si: 0.3-1.5%, Mn: 0.5-2.5
%, P: 0.100% or less, S: 0.030% or less, T
i:48/14×N~2×48/14×N%, Nb:
93/12×C% or more, and Nb≧Ti%, N: 0.0
040% or less, Al: 0.010-0.090%,
After hot rolling a steel having a steel composition consisting of the balance Fe and unavoidable impurities in a temperature range of Ar3 point or higher,
It is rolled up at 0°C or below, then cold-rolled to 800°C.
It is characterized by being annealed in the temperature range from °C to Ac3 point,
A method for producing high-strength cold-rolled steel sheets for deep drawing that have excellent resistance to secondary processing brittleness.